Despite being identified in the 1999 Institute of Medicine report 'To Err is Human' as a central facet of patient safety and quality improvement, diagnostic errors (missed, delayed or incorrect diagnoses) remain understudied and cause significant morbidity and mortality. Limited pediatric-focused research on diagnostic errors highlights the significance of the problem in children: 54% of pediatricians report making a diagnostic error at least monthly and 45% report making a diagnostic error that harms patients at least annually. Errors in diagnosis were the most prevalent cause in closed pediatric malpractice claims. Few studies investigate pediatric diagnostic errors and even fewer attempt to reduce them. This grant proposes to conduct a multisite, prospective, stepped wedge cluster randomized trial testing a quality improvement collaborative intervention within the American Academy of Pediatrics' Quality Improvement Innovation Networks (QuIIN) to reduce the incidence of pediatric primary care diagnostic errors. The proposal will focus on 3 specific, high-risk, ambulatory diagnostic errors each representing a unique dimension of diagnostic assessment: evaluation of symptoms (missed diagnosis of adolescent depression), evaluation of signs (missed diagnosis of pediatric elevated blood pressure) and follow-up of diagnostic tests (delayed diagnosis of actionable laboratory results). This proposal specifically responds to the AHRQ Special Emphasis Notice NOT- HS-13-009 designed to improve diagnostic performance in ambulatory care settings, and focuses on children, an AHRQ priority population. We will test whether a quality improvement collaborative, consisting of evidence- based best-practice methodologies, mini-root cause analyses, data sharing, and behavior change techniques, is associated with a reduction in these 3 specific diagnostic error rates in a randomized national group of pediatric primary care practices. This proposal will enhance the epidemiologic understanding of ambulatory pediatric diagnostic errors through baseline data collected from the quality improvement collaborative and the control group. This research will serve as a potential foundation for projects aimed at global reductions of pediatric diagnostic errors across settings and diagnoses.